Non-contacting shaft sealing devices are well known for providing a seal around a shaft where the shaft penetrates a case or housing, the sealing device serving to separate a region at one pressure on the interior of the case from a region at a different pressure on the exterior of the case. Typically, these devices include two seals in face-to-face relationship: a shaft seal mounted for rotation with the shaft and a case seal mounted on the case. Grooves are provided on the surface of the shaft seal, the grooves communicating with the fluid region being sealed and being designed to generate from the sealed fluid a dynamic pressure which acts against the surface of the case seal thereby tending to move the case seal away from the shaft seal. A spring acts against the case seal in opposition to the generated pressure and a balance is established such that during rotation of the shaft seal a thin fluid film maintains the surfaces of the seals in a non-contacting state.
FIG. 17 illustrates the configuration of grooves provided in the surface of one seal of the prior art for generating the dynamic pressure. Dynamic pressure generating grooves b are formed parallel to one another on a sealing end face or surface a of a shaft seal and extend helically in the peripheral direction of the seal in order to generate a dynamic pressure (positive pressure) between the sealing end face a and the sealing end face of the case seal (not shown). As one of the sealing end faces is rotated relative to the other, the back pressure due to the fluid to be sealed acting on the sealing end face of the case seal, and the spring force pressing this sealing end face toward the sealing end face a of the rotary shaft seal may be balanced, and the sealing end faces are held in a non-contacting state with a fluid film interposed between them.
The helical dynamic pressure generating grooves b generate a dynamic pressure only when the rotary shaft or the sealing end face a is rotated in the direction indicated by arrow A. A dynamic pressure is not generated between the shaft seal surface and the case seal surface when the surface a rotates in the reverse direction. Therefore, a seal having the groove configuration shown in FIG. 17 could not be used in rotary machines having a shaft which may rotate in both a forward and a reverse direction.
Furthermore, although plural dynamic pressure generating regions are arranged parallel in the peripheral direction, only one dynamic pressure generating groove b is present in each dynamic pressure generating region, that is, there is only one dynamic pressure generating spot in each dynamic pressure generating region. Therefore, if dust or dirt contained in the fluid to be sealed invades and is deposited in part of the dynamic pressure generating grooves b, the dynamic pressure generated in the entire dynamic pressure generating region including the clogged dynamic pressure generating groove b decreases. In such a case, therefore, the shaft sealing function may be lowered or fail.